1. Field of the Invention
The present invention relates to a power semiconductor device, and particularly relates to a power semiconductor device which improves a molding shape of a resin case while a nut is embedded in a thin portion of the resin case for connecting an external electrode to another semiconductor device.
2. Description of the Prior Art
In general, a power semiconductor device (hereinafter, also referred to as xe2x80x9csemiconductor power modulexe2x80x9d) such as an insulating gate type bipolar transistor (IGBT) transforms an input direct current into an alternating current having arbitrary frequency by utilizing a semiconductor and outputs the alternating current. The power semiconductor devices are used for motor control, an inverter according to various kinds of application, an uninterruptible power supply (UPS), or the like. Usually the semiconductor power module is formed in a shape of a quadrilateral, and various kinds of functional parts are provided on a cooling metal base of a quadrilateral, and an outside of the semiconductor power module is covered with a resin case.
An insulating substrate is fixed to the cooling metal base, and a circuit pattern is fixedly formed on a surface of the insulating substrate. A plurality of semiconductor chips is mounted on the circuit pattern, a terminal of each semiconductor chip is connected to an electrode plate, and the circuit pattern is connected to another electrode plate. These electrode plates are insulated from each other and extended toward the outside of the resin case, and each electrode plate forms a main circuit terminal for external connection. An inside of the resin case is filled with a silicone gel or the like material for protecting the semiconductor chip and other parts.
Thus, an insulating layer or an insulating metallized substrate is provided on the metal base (i.e., radiator plate), the semiconductor chip is soldered onto the insulating layer or the insulating metallized substrate, and the resin case having a resin cover is fixed to the metal base plate with an adhesive or a screw. The electrode portion to be connected to another semiconductor device is formed as an external lead connecting end portion of the main circuit terminal.
In the above-described semiconductor power module, a hexagonal nut is inserted in and fixed to the resin case as a nut for receiving a bolt, which is located in a lower portion of the electrode portion for external lead connection. That is to say, these hexagonal nuts are inserted in and fixed to hexagonal nut insertion holes, which are formed in the thin portions of the resin case, at a backside of the external lead connecting end portion side of the main circuit terminal, and are arranged adjacent to each other. Electric connection is performed in such a manner that, each of the hexagonal nut insertion hole is formed corresponding to the shape of each hexagonal nut to thereby insert the hexagonal nut when the resin case is molded, and an upper portion of the electrode is folded, and the hexagonal nut is clamped to another semiconductor device with a bolt or the like.
On the other hand, in the clamping construction of the resin case of the conventional semiconductor power module, there is disclosed a technique in which an S-shaped metal cylinder is inserted in a screw portion for a structure preventing a crack caused by stress concentration (for example, see Patent Document 1: Japanese Patent Laid-Open No. H9-129823.
There is disclosed another construction preventing the crack by a structure in which a notch portion of a corner portion is rounded in the resin case (for example, see Patent Document 2: Japanese Patent Laid-Open No. H7-66310).
There is disclosed further another construction in which a slit-shaped groove is formed in a resin member bolted with a screw and destruction of the resin case is prevented by utilizing a partial elastic function (for example, see Patent Document 3: Japanese Patent Laid-Open No. 2000-74016).
However, in the conventional semiconductor power module, because of a problem such as interchangeability of an outside dimension, it is necessary to reduce a thickness of the resin case in which the hexagonal nut for receiving a clamping bolt is inserted to be fixed. When the hexagonal nut in the thin portion of the resin case is clamped to another semiconductor device with a bolt, the stress is concentrated on the corner portion of the hexagonal nut between the hexagonal nut inserted in the resin case and the resin case, and there is a problem such that the thin portion of the resin case cannot withstand the stress during the clamping and there occurs a crack in the thin portion of the resin case.
In order to prevent the resin case from cracking at the corner portion, it is necessary in the conventional construction to increase a thickness of the resin case, which results in an increase in the case size to be a problem. In order to prevent an external screw and the nut from loosening, a member such as a spring washer is required, which results in that the number of parts is increased, and the assembly and disassembly operation are problematically increased.
On the other hand, the construction disclosed in Patent Document 1 is the technique which prevents the crack from occurring in the resin case, by supporting a force of the bolting with the S-shaped metal cylinder. However, it is necessary that the S-shape metal cylinder is produced with a forming process which is more complicated than the usual polygonal nut, and therefore it is not easy to embed and fix the S-shaped metal cylinder to the resin case unlike nut insertion. Accordingly, a projecting portion from the resin case is provided, so that the thickness after the assembly is also increased as a whole.
The construction disclosed in Patent Document 2 is the technique in which a chamfer portion is formed at the corner portion of an aperture end face in the resin case so that the stress concentration on the corner portion is prevented when the inserted electrode terminal is folded. However, it is not disclosed in this document to prevent the generation of the crack which is caused by the concentration of the clamping stress of the polygonal nut on the corner portion of the nut when clamped with a bolt.
The construction disclosed in Patent Document 3 is to include a slit extending radially toward a lower hole formed in a base board. However, it is not disclosed in this document to improve a corresponding position of the corner portion of the nut in the nut insertion hole formed in a resin thin film member.
In view of the foregoing, it is an essential object of the present invention to provide a power semiconductor device in which the crack in the resin case can be prevented from generating at the corner portion of the hexagonal nut, while suppressing the increase in thickness of the resin case, and miniaturization of the resin case can be achieved.
It is another object of the present invention to provide a power semiconductor device in which the external screw and the nut are prevented from loosening with the easy construction, the spring washer is not required, the number of parts is decreased, assembly and disassembly are quite convenient, and productivity is excellent.
In order to achieve the objects mentioned above, the present invention provides a power semiconductor device which includes a resin case for covering and protecting an outer surface of the power semiconductor device. The resin case has a thin portion integrally outward projected therefrom. An external connecting terminal portion of a main circuit terminal extends outward from the resin case, wherein the thin portion of the resin case is formed on a backside of the external connecting terminal portion. A polygonal nut for receiving a clamping bolt is securely inserted in a nut insertion hole which is formed in the thin portion of the resin case. Thus, the polygonal nut is engaged with an inner surface of the nut insertion hole. In this construction, the inner surface of the nut insertion hole has a round-shaped notch concave portion formed at a position confronting to a corresponding corner portion of the polygonal nut so that the corner portion of the polygonal nut is not in contact with a resin case member.